It's hard to tell, and you'll have to figure it out largely yourself, so think what could be it. First the razor can cut arm hair - that's a good sign. Is the facial hair so much different than arm hair? Then you already have experience with a DE.

It could be helpful to draw parallels between DE and straight razor.

So, may be take a DE blade and see how it cuts arm hair - do you see similarities, big differences?
Next consider that the difference between a DE and a straight razor shaving isn't a big one - you draw a blade across your skin and it cuts the hairs. With a DE you have a special holder, but that's still only a holder. May be try the armhair again - shave a small patch with your DE carefully paying attention to how the blade is angled with respect to your skin, how much pressure is exerted, then take the straight razor and try shaving a small patch right next to the first one angling the razor the same way the DE blade was angled, moving it the same way, it should have similar pressure against the skin. If that works on your arm it may be easier to replicate the same thing on your face (use a flat part below a sideburn).

You could also look for somebody in your area who could help you in person.